The Sy reference point is described in 3GPP TS 29.219 version 11.2.0 Release 11.
As the demand increases for varying types of applications within mobile telecommunications networks, service providers must constantly upgrade their systems in order to reliably provide this expanded functionality. What was once a system designed simply for voice communication has grown into an all-purpose network access point, providing access to a myriad of applications including text messaging, multimedia streaming, and general Internet access. In order to support such applications, providers have built new networks on top of their existing voice networks, leading to a less-than-elegant solution. As seen in second and third generation networks, voice services must be carried over dedicated voice channels and directed toward a circuit-switched core, while other service communications are transmitted according to the Internet Protocol (IP) and directed toward a different, packet-switched core. This led to unique problems regarding application provision, metering and charging, and quality of experience (QoE) assurance.
In an effort to simplify the dual core approach of the second and third generations, the 3rd Generation Partnership Project (3GPP) has recommended a new network scheme it terms “Long Term Evolution” (LTE). In an LTE network, all communications are carried over an IP channel from user equipment (UE) to an all-IP core called the Evolved Packet Core (EPC). The EPC then provides gateway access to other networks while ensuring an acceptable QoE and charging a subscriber for their particular network activity.
The 3GPP generally describes the components of the EPC and their interactions with each other in a number of technical specifications. Specifically, 3GPP TS 29.212, 3GPP TS 29.213, and 3GPP TS 29.219, describe the Policy and Charging Rules Function (PCRF), Policy and Charging Enforcement Function (PCEF), and the Sy reference point. These specifications further provide some guidance as to how these elements interact in order to provide reliable data services and charge subscribers for use thereof.
The PCRF uses the policy counter statuses relating to subscriber spending, received from the OCS via the Sy reference point as input to its policy decisions, e.g., downgrade the Quality of Service (QoS) (e.g. APN-AMBR) or modify the PCC/QoS/ADC Rules. In this context therefore, improved flexibility of online charging system is highly desirable.